SRID = Spatial Reference Id.
A SRID is a number choosen to represent a spatial reference system. The SRID uniquely identifies this system.
Each system is basically composed of:
Geographic Spatial Reference Systems = Ellipsoid + Datum;
Projected Spatial Reference Systems = Geographic Spatial Reference System + Projection Information;
A lot of people have problems with projections and spatial reference systems, but once you get used to it and start picturing the light bulb and the grid, everything is easier.
Million dollar question
What is the best SRID for my project?
Million dollar answer
It depends ;). You need to identify your needs before start developing and you need to consider the data already available. If in doubt, use the most common SRID in the data available.
PS: the source for the image is the fabulous ESRI book, Understanding Map Projections.